Image transfer (coherent) optical fiber bundles have been used to inspect the interior cavities of the body for the diagnosis and monitoring of disease. They can also be used as the image optical transfer path component in confocal systems. Unlike in the case of a vibrating single fiber, the cores in the tips of the bundle are not acting as confocal pinholes; this function is provided by a spatial filter or filters, such as in the form of a pinhole, associated with a scanning mechanism outside the subject.
Existing fiber bundle confocal systems generally scan a single laser point in a raster pattern on the surface of the tips of the fibers of the bundle, using X and Y mirrors to effect the scan, although the same principle also works with line-scan, Nipkow disc or other confocal systems.
FIG. 1 is a schematic view of a basic point-scanning laser confocal system 10 of the background art that employs a coherent fiber bundle as an image transfer element. A beam of light 12 from a laser 14 shines on a beam-splitter 16 and a portion 18 of the light 12 is reflected onto an x-y beam-scanner 20. The scanned beam 22 passes through lens 24 to focus to a spot at the entry of core 26 of a respective optic fiber 28 on the polished proximal face 30 of the coherent fiber bundle 32. The light passes down the fiber 28 to the distal end 34 of fiber 28, from which it emerges as a divergent beam 36 that is completely captured by collimating lens 38. This light proceeds as a parallel, collimated beam 40 to an objective lens 42 that brings it to a focus as a diffraction-limited Gaussian waist 44 within the specimen under examination. If a portion of the specimen within that waist region 44 fluoresces or reflects light then that light travels in the reverse direction (via the same path as the excitation light described above) until it reaches beam-splitter 16. Some of this return light exits the beam-splitter 16 as a beam 46 and is focussed by lens 48 to a diffraction limited Gaussian waist 50 bounded by a spatial filter pinhole 52. The light passing through pinhole 52 impinges on a photodetector (such as a PMT) 54 and generates an electrical signal. This signal is fed into a bitmap in a frame-store 56 and is displayed as a point 58 on the screen of a monitor 60.
A scan generator 62 shifts the x-y beam-scanner 20 and hence beam 22 to a new path 64 so that the light travels through a different fiber 66 (at a considerable distance from the first fiber 28), illuminating another portion of the specimen at a different Gaussian waist 68; this portion is displayed on the screen of monitor 60 at 70. The scan generator 62 also provides an output signal to frame-store 56 so that frame-store 56 can assign the correct instantaneous x and y coordinates to the signal received from photodetector 54. Ultimately system 10 builds up a final image 72.
Coherent bundles have the major advantage in endoscopy of eliminating any need for a mechanical scanning mechanism within the subject. However, as each fiber core is discrete and separate, with six neighbouring fibers, there is the risk of undersampling and a hexagonal honeycomb overlay is imposed on the ultimate images.
Further, the fiber bundle loses a considerable amount of light, which is further compounded by the undersampling, for the following reasons. If the fiber cores were touching and were arranged in a square array, that is, centred on a rectangular grid, then from information theory principles the figure for linear undersampling in one row should be 2.3 (the Nyquist number) and the a real undersampling 2.32. In fact the cores in a fiber bundle are hexagonally close packed, so that the next row of fibers is actually 0.7071 diameters below the first row and the figure for undersampling in this direction is 2.3×0.7071=1.61. This makes the average figure for (linear) undersampling slightly less than 2 (or in terms of areal undersampling, somewhat less than 4).
In this example, if a purely geometric analysis is performed, approximately 9% of the light falling on a hexagonally close packed array of circular fibers will pass into the gaps between the fibers. However the fiber cores are separated by the combined cladding of each pair of adjacent fibers and, as described above, this cladding must be of finite thickness to avoid photon tunneling between the cores; such tunneling would otherwise cause optical crosstalk and image degradation. Photons which arrive at the bundle tip in the cladding region are either absorbed by the cladding within a short distance of the tip or are diverted at such a high angle that they are not guided but traverse across the cores to be absorbed at the outer sheath of the bundle. The cladding does not act as a funnel at the tip for the photons, there has to be a “dead zone” in between the cores to stop tunneling.
Apart from causing light loss, this dead-zone also adds to the undersampling; if the laser input beam were carefully matched to a single fiber in a polished tip of a bundle, it is possible to launch over 80% of light to reach the other end of the bundle. The loss is from tip reflections (˜8%), Raleigh scattering, and glass absorption.
If the laser beam is defocused to cover several cores, the transmission typically drops to ˜20%. This implies by geometric arguments that the dead zone makes up around 75% of the polished tip area. It follows from this that the cores are effectively separated by a full core diameter of cladding and that the linear undersampling figure is approximately double that for cores in close contact. This number will vary slightly with wavelength, bundle type and beam profile. (As a separate issue the dead area also implies that 75% of the scanning acquisition time will not be used, unless the laser spot is scanned along the lines of cores only.)
FIG. 2 is another schematic view of the background art system 10 of FIG. 1, and illustrates under-sampling resulting from the discrete nature of the separate fibers of the bundle 32 when using the illustrated system. For the purposes of explanation the illumination of the specimen is shown as being carried out (successively) by two adjacent fibers 80a and 80b. The respective Gaussian waists 82a and 82b of the light focussed by the objective lens 42 do not overlap. Hence respective portions of the specimen in Gaussian waists 82a and 82b will be imaged as 84a and 84b on the screen of monitor 60, but a portion of the specimen at 86 between Gaussian waists 82a and 82b will not be imaged. This constitutes a high degree of under-sampling.
The most obvious feature of fiber bundle images is the reticulated or hexagonal pattern overlay. In some approaches this is removed by acquiring a (highly oversampled) image of the bundle tip and using it to subtract pixel for pixel from the raw images. The pattern can also be removed by deliberately blurring the image or by filter transform processing. These methods improve viewability but at the cost of some information.
To obtain full resolution with maximum light efficiency, however, it would be necessary to sample at points between the discrete core positions shown. The vibrating tip fiber system is able to sample at as many points as desired during the scan.
Another existing approach avoids undersampling and attains full resolution potential as follows. If the numerical aperture (NA) of the fibers in the bundle is matched to the back NA of the lens (as is done in a scanning tip system), then the optical efficiency is at a maximum but the image is undersampled. Pixel intensity values for points building up the image can only be obtained from one fiber and then from the next adjacent fiber core, but the Nyquist sampling criterion requires measurements from points in between. The finite cladding thickness needed to prevent evanescent coupling and cross-talk (i.e. light leakage) between adjacent fibers further separates the cores, and the total linear undersampling value is then close to 4. Full optical resolution can be obtained by synchronized mechanical movement (“dithering”) of the fiber bundle tips at both ends by a few core diameters in X and Y during image acquisition. This allows intensities of pixels to be obtained between the static core positions and eliminate the hexagonal honeycomb overlay pattern of the close packed fibers. From the discussion above it can be seen that it would be necessary to integrate approximately 16 scans (i.e. the square of the linear undersampling figure) to produce one image with full resolution. Mechanical scanning at the distal tip can be effected with piezo actuator elements of the type used to shift a CCD or CMOS chip in a digital camera, to increase resolution or to act as image stabilizers. At the proximal (i.e. laser source) end of the bundle an identical piezomechanical system can be used, or an equivalent result could be achieved in the computer by a shift in the frame register of the image.
FIG. 3 is a schematic view illustrating this background art technique for removing under-sampling by simultaneously dithering proximal and distal tips of the fiber bundle, and thereby obtaining samples at intermediate positions. The system 90 of FIG. 3 is generally like laser confocal system 10 of FIGS. 1 and 2, and like reference numerals have been used to identify like features. In addition, system 90 includes mini x actuators 92a, 92b (at the proximal and distal tips of the fiber bundle 32 respectively) controlled by controller 94 and mini y actuators 96a, 96b (at the proximal and distal tips of the fiber bundle 32 respectively) controlled by controller 98; these mini x and y actuators 92a, 92b, 96a, 96b simultaneously dither the proximal and distal tips of fiber bundle 32. When combined with the effects of beam-scanner 20, imagining can thereby be performed at intermediate positions.
Another existing method to obtain full resolution is to deliberately mismatch the fiber NA and the lens back-NA. A fiber bundle with smaller, high NA cores or a longer focal length collimating lens is used, so that the excitation light overfills the focussing lens. With this approach, the Airy discs projected within the specimen from adjacent fibers overlap, thus allowing the specimen fluorescence to be sampled at intermediate positions and satisfy the Nyquist criterion. The confocal light returning from each of these points (which are acting as sources) in the specimen projects its own Airy disc onto several fiber cores, most of which overlap from one pixel to the next. The return confocal pinhole at the proximal end must then accept light from this cluster of cores.
FIGS. 4A and 5 are schematic views of a background art system 100 in which the distal lens is highly overfilled. System 100 can give full resolution by allowing sampling at the Nyquist criterion intervals. In FIG. 4A the optical system is generally identical with that of FIG. 1, and like reference numerals have been used to identify like features. However, system 100 includes a collimating lens 102 of greater focal length than that of FIG. 1. This means that the distal lens assembly (comprising collimating lens 102 and objective lens 42) is further from the distal tip of fiber bundle 32, and the Gaussian waists 104 (which are identical in size with those in FIG. 2) now overlap and give proper sampling within the specimen being observed.
For clarity the illumination of the specimen is shown in FIG. 4A as being carried out by two adjacent fiber cores 106a, 106b in the fiber bundle 32. This does not occur simultaneously.
FIG. 4B is an enlarged view of region 108 of FIG. 4A. As is more apparent in this detail, Gaussian waist 110a resulting from the light transmitted along core 106a overlaps Gaussian waist 110b resulting from the light transmitted along core 106b. 
FIG. 5 depicts the same optical arrangement as that of FIG. 4A but showing the return light rays from one point in the specimen. The Airy disc of the return light now falls on a cluster fiber tips 112 of seven fibers 114 at the distal end of the bundle 32. (For clarity, only the three fiber tips in the central plane of the cluster are shown.) The pinhole 52 is enlarged to allow the light emitted from the seven proximal cores 116 to pass to the photodetector 54. This gives full resolution but with a worse signal/noise ratio owing to the wasting of excitation light from the overfilling and the loss of return light in the cladding.
FIG. 6 is a schematic view of another background art system 120, similar to system 10 of FIG. 1 and like reference numerals have been used to identify like features. However, distal lenses (cf. lenses 38 and 42 in FIG. 1) are not used; instead, the distal face 122 (which is polished) of the fiber bundle 32 directly touches the tissue 124 to be imaged. Excitation light passes along a single fiber 28 of the bundle 32 and light from components 126 of the tissue 124 close to the distal face 122 returns back along the same fiber 28, passes through the beam-splitter 16 and pinhole 52 to a single photodetector 54, and is imaged at 128 on the screen of monitor 60. Similarly (though not simultaneously), excitation light transmitted by another fiber 66 illuminates another portion 130 of the tissue 124, and is ultimately imaged at 132 on the screen of monitor 60. A complete, contact microscopy image is eventually generated. The signal to noise ratio of the image, however, is degraded by Raman, Raleigh and Fresnel noise, and is affected by dirt on the polished bundle face 122.
However, in order to obtain full optical resolution using available bundles and the above techniques that employ distal tip lenses, over 95% of the laser excitation light is discarded. The mismatch also means that, in the return direction, the cladding absorbs about 75% of the return confocal light as the mode fields no longer match. In order to compensate for the poor light budget, the input power must be increased, but this increases the level of Raman scattering within the bundle and hence degrades the S/N ratio.
Hence, both the signal is reduced and the higher laser power generates correspondingly more noise within the fiber cores. Indeed, if full optical resolution with a bundle system is desired in existing systems, the laser power required to achieve the same level of fluorescent return signal will be 50 times greater. That is, the optical efficiency will be much less than 5% (i.e. 1/16th of 25%) of that of a single fiber tip scanner.
This degrades the S/N ratio in three ways:                1) Statistical noise due to photon arrival variation is worse if laser power is limited by medical safety requirement;        2) Noise from Raman scattering in the fiber core is very much worse; and        3) Fluorescence saturates (and fluorescent return tails off) while Raman generation increases linearly with laser intensity. The 25% confocal return figure pushes the fluorophore more into the saturated region.        
Of these the noise from Raman scattering is generally the most significant. Spectral filtering can be used to remove some of this noise, but Raman lines from glass are broad because of the range of bond energies in the liquid glass. Methods of removing Raman interference do exist. For example, Raman noise is generated instantaneously so can be removed from the fluorescent signal using a FLIM system, but this requires pulsed lasers and time gated detectors.